1. Field of the Invention
The present invention relates to an endoscope, and more particularly to an endoscope which has as an image pick-up device a plate like image sensor disposed within a viewing head thereof.
2. Description of the Prior Art
In addition to endoscopes having an optical fiber bundle as its image guide means, there have been proposed TV endoscopes of a type having as an image pick-up device an image sensor such as a charge coupled device (CCD) which comprises a great number of small photosensitive elements (pixels) arranged in a matrix. Such TV endoscopes, which are far better than endoscopes heretofore used considering from the standpoints of durability, the effect of video signal management and production cost, are in the improvement stage for practical application.
Image sensors now available, although which are miniaturized owing to recent progress in semiconductor production technology, specifically to the increase of integrating density, are too large in size to be incorporated within endoscopes. That is because endoscopes should be made with a small diameter for the purpose of insertion into a cavity of a living body. For example, a gastro-endoscope has its outer diameter of approximately ten (10) mm at the most. If image sensors now available, are miniaturized to a considerable degree suitable for being incorporated into endoscopes, there is left pending the problem of resolution. The requirement of improving resolution will require an image sensor to have an increased number of pixels, resulting in an increase in size.
With these points in background, it becomes very important how to arrange both an image sensor and an objective lens assembly forming directly optical image on the image sensor in a narrow space inside the viewing head of an endoscope. There will be considerations, one being that if an endoscope is of a front view type, an image sensor plate is placed parallel to a plane intersecting perpendicular to the longitudinal axis of the viewing head, and the other being that if an endoscope is of a side view type, an image sensor plate is placed close to the inside wall and along the longitudinal axis of the viewing head, It is, however, very difficult to make such an arrangement. The reason is that if it is a front view type of endoscope, considering a cross section of the viewing head, the image sensor plate occupies the majority of the cross section, making it difficult to incorporate essential elements such as light guide means, a forceps channel and an air and water supplying channel in the viewing head. In case of a side view type of endoscope, an objective lens assembly also occupies a cross section of the viewing head because it is placed close to the inside wall opposite to the image sensor plate so as to form an optical image onto the image sensor, resulting in the same difficulty as mentioned above. In addition, it is foreseen to deform the shape of either the image sensor plate or the viewing head when placing the plate like image sensor close to the inside wall of the viewing head having a circular cross section.